Generally in an image formation apparatus, such as a laser printer, the following processes are performed:
a) a uniform distribution of electrical charges is produced on a surface of an electrostatic latent image carrying body;
b) an electrostatic latent image is formed on a charged area of the image carrying body surface by an optical writing means such as a laser beam scanner, an LED (light emitting diode) array, a liquid crystal shutter array or the like;
c) the latent image is developed as a visible image with a developer or toner which is electrically charged to electrostatically adhere to the latent image zone;
d) the developed and charged toner image is electrostatically transferred from the surface of the image carrying body to a recording medium such as a cut sheet of paper; and
e) the transferred toner image is fixed and recorded on the cut sheet of paper by a toner image fixing means such as a heat roller.
Typically the electrostatic latent image carrying body may be an electrophotographic photoreceptor. The receptor is usually formed as a drum, called a photoconductive drum. The photoconductive drum typically has a cylindrical conductive surface and a photoconductive insulating film bonded to the cylindrical conductive surface. In the charging process, an electric discharger, typically called a primary charge roller, is used to produce the charged area on the photoconductive drum. Such a primary charge roller is typically aligned adjacent to and parallel to the photoconductive drum such that it is in contact with the photoconductive drum along substantially the entire length of the primary charge roller. The primary charge roller must contact the photoconductive drum so as to fully charge the surface of the photoconductive drum.
Typically, laser printers include a replaceable cartridge. The cartridge includes the photoconductive drum, the primary charge roller, toner, and a developer roll. The cartridge is replaced periodically as required when a part or parts fail, or the toner has been completely used. A cartridge can typically last as long as 17,000-24,000 copies at 2.5% coverage per copy. One type of premature failure involving the photoconductive drum and the primary charge roller is called knife edge failure or black line short. The edge of the charge roller contacting the photoconductive drum wears a groove into the photoconductive drum such that the surface of the photoconductive drum cannot be evenly charged. This produces a black line on each page that is printed. It can also destroy both the primary charge roller and/or the photoconductive roll, thereby rendering the cartridge useless.
Various attempts to prevent this premature wearing of the photoconductive drum by the edges of the primary charge roller have been made. For example, radii or chamfers have been provided on the edges of the primary charge roller. The present invention eliminates the need for such a radius or chamfer, while preventing knife edge failure.
Numerous primary charge roller configurations are used in the art. Variations in configuration have been made to address different issues with the primary charge roller and the photoconductive drum. For example, U.S. Pat. No. 5,541,711, Kisu et al., issued Jul. 30, 1996, discloses a charging member having an internal cavity. This internal cavity is intended to eliminate noise that can be produced when the photoconductive drum is charged. This cavity is defined by the metal shaft about which the charging roller rotates and the outer surface of the charging roll. The internal cavity extends substantially the entire length of the charging roller and occupies a substantial volume of the charging roller. The cavity described in the patent addresses a different problem than the knife edge failure addressed by the present invention. Furthermore, the patent discloses an internal cavity completely enclosed within the charge roller. In contrast, the present invention requires a gap that is defined on three sides, but open on the fourth side. The present invention does not encompass a primary charge roller configuration wherein the fourth side of the gap is also enclosed.